The present invention relates to combustors for combustion turbines and more particularly to combustors capable of reduced emissions of nitrogen oxides, NOx.
It is known that NOx formation increases with increasing flame temperature and with increasing residence time in the combustor. It is therefore theoretically possible to reduce NOx emissions from a combustor by reducing the flame temperature and/or the time at which the reacting gases remain at the peak temperatures. In practice, however, this is difficult to achieve because of the turbulent diffusion flame characteristics of present day combustion turbine combustors. In such combustors, the combustion takes place in a thin layer surrounding the evaporating liquid fuel droplets at a fuel/air equivalence ratio near unity regardless of the overall reaction zone equivalence ratio. Since this is the condition which results in the highest flame temperature, relatively large amounts of NOx are produced. As a result, the conventional single stage, single fuel nozzle spray atomized combustors may not meet newly established emission standards regardless of how lean the nominal reaction zone equivalence ratio is maintained.
It is also known that significant reductions in NOx emissions can be achieved by injection of water or steam into the combustor reaction zone. However, such injection has many disadvantages including an increase in system complexity and high water treatment costs.
The problem of realizing low NOx emissions develops further complexity where it is necessary to meet other combustion design criteria. Among such criteria are those of good ignition qualities, good crossfiring capability, stability over the entire load range, large turndown ratio, low traverse number, long life and ability to operate safely and reliably.
Factors which result in the formation of NOx from fuel bound nitrogen and air nitrogen are known and efforts have been made to adapt various combustor structures in light of these factors. For example, U.S. Pat. Nos. 2,999,359; 3,048,014; 3,946,533; 3,958,413; 3,958,416 and 3,973,395 describe various combustor structures for use in combustion turbines. These combustors, however, have either not been adaptable for use on stationary combustion turbines or have been inadequate for other reasons such as cost, complexity, unreliability or unacceptable performance characteristics.
In copending patent application Ser. No. 3,016 filed Jan. 12, 1979 by R. A. Farrell et al and of common assignee, a dual stage low NOx combustor for a stationary combustion turbine is described. This application contains subject matter related to the Farrell et al application and the invention described herein is an improvement upon that invention.
It is an object of this invention to provide a dual stage low NOx combustor for a stationary combustion turbine which operates over the entire turbine cycle with substantially reduced pollutant emissions, principally NOx and carbon monoxide. It is a further object of this invention to provide a method and apparatus for producing low emissions of NOx and carbon monoxide from a combustion turbine combustor characterized by good ignition and crossfiring qualities, stability over the load range, large turndown ratio, low traverse number, long life and safe and reliable operation. Other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description taken in conjunction with the accompanying drawings.